Distinguish between ostracoderms and placoderms. What important evolutionary advances did each contribute to vertebrate evolution? What are conodonts?

The major difference between ostracoderms and placoderms is that ostracoderms are jawless, while placoderms were jawed vertebrates, and were among the first jawed vertebrates.
Fossil evidence of Ostracoderms pushes the origin of vertebrates back to at least the Cambrian. Otracoderms also developed a new way of feeding. Rather than filter feeding, they sucked water into their pharynx by muscular pumping. This suggests that they may have been mobile predators. Ostracoderms also contributed paired pectoral fins.
Conodonts are extinct vertebrates that were very hard to identify. They are small and tooth-like, and have all of the characteristics of vertebrates. They are important in understanding the origin of vertebrates.